1. Field of the Invention
This invention generally relates to signal communications and, more particularly, to a training signal for use in a multi-channel Optical Transport Network (OTN) protocol.
2. Description of the Related Art
Signals may be communicated over parallel media using an inverse multiplexing scheme to concatenate channels. Conventionally, a virtual concatenation (VCAT) protocol uses an inverse multiplexing technique to split a signal into a payload of multiple Synchronized Optical Networking (SONET)/synchronous digital Hierarch (SDH) signals, which may be transmitted over independent routes. SONET/SDH protocols have been optimized for the transport of voice channels. VCAT uses multiple SONET/SDH containers to carry streams of data that would otherwise not fit in standard SONET/SDH containers, and is specified in ITU-T Recommendations G.707 (2007) and G.783 (2006).
The bandwidth is split equally among a set number of sub paths called Virtual Tributaries (VT). Several Virtual Tributaries form part of a Virtual Concatenation Group (VCG). The spawning of Virtual Tributaries to transport data across a VCAT-enabled network permits alternate paths to be used, which can be useful in congested networks. The Virtual Concatenation protocol uses byte-interleaving. For example, a Gigabit Ethernet (n, 1 Gb/s) may be distributed across (i) STS-nc VT's, where each VCG member carries a bandwidth equivalent of V=n/k [bits/second], n=1 Gb, and k=i. VCAT can also be used to concatenate multiple OTN channels using the same mechanism defined for SONET/SDH.
VCAT of multiple OTN channels will be used for transport of client signals with data rate of 100 Gbps, but this requires the monitoring of multiple channels and the use of additional bandwidth for the redundant overhead. The VCAT mechanism is suitable for use in physically parallel channels (e.g., different fiber). However, additional buffers are required for overcoming the skew (differential delay) between the different media.
100 gigabit per second (Gbps) OTN protocols are being developed where the OTN streams are carried in a “parallel fashion” at a bit-rate greater than 100 Gbps. Until recently, optical signals have been carried using 1 bit per symbol modulation techniques. However, the need for faster signal speeds means that return-to zero (RZ), non-return-to-zero (NRZ), and phase-shift keying (PSK) modulations techniques (1 bit per symbol) are no longer suitable.
A proposed solution for the transmission of high speed signals ( 40/100 Gbps and above) uses a combination of multiplexed orthogonal optical polarization and 2 bits per symbol modulation techniques on each of the two polarizations. This multiplexing and modulation scheme, polarization multiplexed (PM)-QPSK, is a combination of serial and parallel communication. The communication is not entirely serial because the two polarizations are not completely synchronized, but it is not entirely parallel because the physical medium and lambda are the same. However, OTN signals were originally defined for use in serial transmissions only. That is, OTN signals do not have characteristics inherent for efficient transmission and reception over “semi-parallel” channels. Since the semi-parallel channels may experience skew (differential delay), the recovery of a serial stream from parallel streams will require new techniques. While VCAT could potentially be used to solve this problem, a solution based on virtual concatenations would require a higher overall rate for the optical transmission, and multiple entities to be monitored.
It would be advantageous if a technique could be developed for more simply addressing the recovery of the OTN serial stream for high speed ( 40/100 Gbps and above) parallel stream networks.